Core-shell particles with catalytic activity

ABSTRACT

The present invention pertains to novel core-shell particles comprising a core of iron oxide and a shell of cobalt oxide, characterized in that they are spherical with a number average diameter, as measured by TEM, of between 1 and 20 nm. This invention is also directed to their uses in the manufacture of a catalyst, and to the method for preparing these particles, by precipitating cobalt oxide onto magnetite or hematite particles which are themselves precipitated from Fe(III) and optionally Fe(II) salts.

FIELD OF THE INVENTION

The present invention pertains to novel core-shell particles, to themethod for preparing same, and to their uses in the manufacture of acatalyst.

BACKGROUND OF THE INVENTION

Many chemical and petrochemical reactions are performed in the presenceof catalysts comprising transition metals, such as platinum or cobalt,which are very expensive. Attempts have thus been made to proposeparticles comprising one of these transition metals mixed with a lessexpensive metal.

As far as cobalt is concerned, it has been suggested in EP 0 261 870 touse in the conversion of syngas to hydrocarbons (in the so-calledFischer-Tropsch process), after reductive activation, a catalyst madefrom core-shell particles having a core of zinc oxide and a shellcomprising cobalt oxide. These particles may be produced either byco-precipitation of insoluble thermally decomposable compounds of zincand cobalt, or by precipitation of an insoluble thermally decomposablecompound of cobalt in the presence of zinc oxide. Typically, theinsoluble thermally decomposable compounds are formed from metal oxidesobtained by adding a precipitant such as a base to an aqueous solutionof the corresponding metal salts. The drawback of the co-precipitationmethod is that the size of the particles cannot be properly controlled,which in turn detrimentally affects the conversion rate and selectivityof the catalytic process in which these particles are used.

Moreover, the catalytic activity of the particles obtained according tothese two methods need to be improved. Another method to preparecore-shell catalysts has been proposed in U.S. Pat. No. 7,422,995 and isreferred to as the layer-by-layer (or “LBL”) method. Again, thecatalytic activity of the particles can be improved. Moreover, itinvolves several steps and the use of a surfactant to anchor the cobaltlayer to the chemically inert core coated with a zinc oxide layer, whichincreases the cost of this method.

Other core-shell materials have been proposed for catalytic applications(J. BAO et al., Angewandte Chemie, International Edition in English,Vol. 47, pp. 353-356, 2008 ; J. M. BADANO et al., Applied Catalysis A.,Vol. 390, pp. 166-174, 2010). They include an active phase, generally atransition metal, which is set as the core of the composite particle.This active core is then coated with a protective shell, such asmesoporous silica, titania or carbon nanotubes. Covering the activephase with a protective shell can prevent sintering, while allowingreactants and products to diffuse through the catalyst. However, thesystem is severely affected by the diffusion limitation across theshell. Examples of such core-shell catalysts have been used inFischer-Tropsch reactions (R. XIE et al., Catalysis Communications, Vol.12, pp. 380-383 and pp. 589-592, 2011).

Therefore, there remains the need to provide a cost-effective method forpreparing core-shell metal particles intended to be used in themanufacture of a catalyst which comprises a much lower amount of cobaltthan known catalysts while having at least the same catalytic activity.

This need has been satisfied by a novel method which leads to specificparticles having a core of iron oxide and a shell comprising cobalt.Moreover, to the inventors' knowledge, these particles have never beendescribed before.

Specifically, core-shell particles having a core of iron oxide and ashell comprising cobalt, and other cobalt-doped iron oxide particles,have already been described in various documents such as U.S. Pat. No.6,080,233, U.S. Pat. No. 5,512,317, U.S. Pat. No. 5,484,628, U.S. Pat.No. 5,183,709, U.S. Pat. No. 4,276,183, U.S. Pat. No. 4,226,909, U.S.Pat. No. 4,420,537 and U.S. Pat. No. 3,573,980. These particles areintended to be used as magnetisable particles in magnetic recordingtapes which require both a high coercivity and a good orientation ratioof the particles in a binder. These particles have an acicular shape,resulting from their preparation processes. These processes use aciculariron oxides as starting materials, which are treated in a basic aqueoussolution so as to form a core of magnetite, berthollide or γ-Fe₂O₃,having a size of more than 20 nm and most often more than 100 nm. Acobalt salt (and optionally other metallic salts or a silicate) is addedto the iron compound either before or after the formation of the core,so as to result in a shell comprising cobalt and optionally iron orchromium (and optionally a silicate). Similar processes have beendisclosed by A. E. Berlowitz et al in IEEE Transactions on Magnetics,Vol. 24, No. 6, November 1988, by M. KISHIMOTO et al in IEEETransactions on Magnetics, Vol. Mag-21, No. 6, November 1985, by H.SESIGUR et al. in Materials Research Bulletin, Vol. 31, No. 12,pp.1581-1586, 1996 and by K. SAKAI et al. in J. Appl. Crystal., Vol. 34,pp. 102-107, 2001.

The above prior art does not suggest that spherical particles, having acore of iron oxide of less than 100 nm, and even less than 20 nm, and ashell comprising cobalt, can be produced. Moreover, these documents donot suggest the simple and inexpensive method of this invention, whichmay be carried out to produce these particles with a controlled shellthickness.

SUMMARY OF THE INVENTION

In one aspect, the present invention is thus directed to a method forthe preparation of spherical core-shell particles, comprising thesuccessive steps consisting of:

-   (a) preparing an aqueous solution comprising a ferric salt, at a    temperature of less than 50° C.;-   (b) adding at least one base to said solution, so as to obtain a    suspension of iron oxide particles having a pH value of from 10 to    14;-   (c) washing the suspension;-   (d) adding a strong acid to the washed suspension to peptize it;-   (e) reacting at least one base with said peptized suspension, until    the pH reaches a value from 10 to 14, at a temperature of from 50 to    95° C.,-   (f) adding a cobalt salt to the heated suspension in order to obtain    spherical particles having a core of iron oxide and a shell    comprising cobalt oxide.

It is understood that the above method may comprise other preliminary,intermediate or subsequent steps, as long as they do not impair thestructure and properties of the core-shell particles obtained.

In another aspect, this invention pertains to the core-shell particleswhich may be obtained according to the above method.

In yet another aspect, this invention is directed to core-shellparticles comprising a core of iron oxide and a shell of cobalt oxide,characterized in that they are spherical with a number average diameter,as measured by TEM, of between 1 and 20 nm. In a preferred embodiment,the core consists in magnetite or hematite and the shell consists incobalt oxide.

In yet another aspect, this invention pertains to the use of thesecore-shell particles to manufacture a catalyst.

In still another aspect, this invention pertains to a catalystcomprising an inert porous carrier containing core-shell particles asdefined above.

DETAILED DESCRIPTION

This invention will now be described in further details. In thefollowing description, the expression “comprised between” should beunderstood to designate the range of values identified, including thelower and upper bounds.

The novel method of this invention for the preparation of core-shellparticles mainly involves the precipitation of iron oxide particles fromiron salts, so as to form a magnetite (Fe₃O₄) or hematite (Fe₂O₃) core,followed by the addition of a cobalt salt, in a hot basic medium, inorder to precipitate a cobalt oxide shell (Co₃O₄) around said core.

Specifically, in the first step of this method, an aqueous solution isprepared, which comprises a ferric or

Fe(III) salt, optionally mixed with a ferrous or Fe(II) salt in a molarratio of Fe(III) to Fe(II) of 2:1. These salts may be independentlychosen from, for instance, nitrate, chloride and hydroxide salts, withchloride salts being preferred. Preferably, sulphate salts will not beused. The solution of these salts in water is maintained at, or broughtto, a temperature of less than 50° C., preferably comprised between 15and 40° C. and more preferably comprised between 20 and 30° C. Thissolution generally comprises no other compound than the above salts,especially no other salt and/or no surfactant.

To this solution is then added at least one base so as to obtain anaqueous suspension of iron oxide(s). This base is preferably ammoniumhydroxide, although other bases such as sodium or potassium hydroxidemay be used. This addition is performed until a pH of from 10 to 14 isreached, generally while stirring the solution.

At this point, one obtains a suspension containing hematite particles(Fe2O3) or magnetite particles (FeO.Fe₂O₃ also designated by Fe₃O₄),depending on whether only Fe(III) or a mixture of Fe(III) and Fe(II) wasused, respectively. This suspension is then washed with water,preferably at 10-40° C., for instance at 10-30° C., so as to remove theexcess ions, before adding a strong acid, such as nitric acid orhydrochloric acid, to the washed suspension. The function of this strongacid is to peptize the suspension, i.e. both destroy loose aggregates ofiron oxide(s) and stabilize the suspension. It may also be useful tocomplete the oxidation of ferric and optionally ferrous salts into ironoxide(s), if necessary, provided that the suspension is heated, forinstance up to 100° C. Usually, this strong acid is added until a pH offrom 3 to 5 is attained.

The iron oxide particles thus obtained have a spherical shape with anumber average diameter, as measured by Transmission Electron Microscopy(TEM), comprised between 1 and 20 nm and preferably between 3 and 15 nm,which has been shown to be dependent on the precipitation pH.

In a further step of the method according to this invention, a base isadded to the suspension of magnetite particles, until a pH of from 10 to14 is reached. This base may be chosen from those listed above, amongwhich sodium hydroxide is preferred. The addition of this base isgenerally performed while stirring the suspension.

This base is reacted with the suspension of iron oxide particles at atemperature of from 50 to 95° C. and preferably comprised between 60 and70° C. The suspension may either be heated first, then reacted with thebase, or first reacted with the base and then heated.

A solution of cobalt salt is then added slowly to this suspension, forinstance at a rate of from 0.1 to 0.5 ml/min. This salt may be chosenfrom cobalt chloride, cobalt nitrate, cobalt sulphate and their hydratesand mixtures, wherein cobalt nitrate is preferred. The addition of thissalt is also usually carried out while stirring the suspension. In thisstep of the method, at least one other metal salt may be added,especially salts of catalytic promoters such as platinum, manganese orruthenium and their mixtures. The amount of cobalt salt, and optionallyother salts, used depends on the thickness of the cobalt oxide shellthat is to be formed around the magnetite core particles.

This step leads to the formation of core-shell particles having a numberaverage diameter, as measured by TEM, comprised between 2 and 30 nm andpreferably between 3 and 15 nm, and a weight ratio of cobalt to ironcomprised between 1 and 70%, preferably from 10 to 65% and morepreferably from 15 to 60%. The lower this ratio, the higher the costsavings compared to usual cobalt particles used in catalysts. This ratioshould however be properly chosen in view of the reaction in which theseparticles are intended to be used, so as to provide the requiredcatalytic activity.

As evident from the foregoing, the method according to this invention issimple, with only a few steps, it need not be carried out under specificconditions, for instance under inert atmosphere. Moreover, this methoddoes neither generate hazardous by-products, nor contaminated waters,and uses standard low-cost chemicals.

As mentioned above, the core-shell particles of this invention arenovel. They comprise a core of iron oxide and a shell of cobalt oxideand they are spherical with a number average diameter, as measured byTEM, of between 1 and 20 nm. As described above, the core preferablyconsists in magnetite or hematite and the shell consists in cobaltoxide. Usually, these particles do not include any other metal oxide.

They may be used to manufacture catalysts involved in Fischer Tropschreactions or in other chemical reactions such as the hydrogenation ofnitriles into primary amines or in hydroformylation processes.

The Fischer-Tropsch process generally comprises the following processsteps. The first step involves reacting a source of carbon (such ascoal, natural gas or biomass) with a source of oxygen (such as steam,air or oxygen) to form a mixture of carbon monoxide and hydrogen,usually referred to as syngas. The second step involves contacting thesyngas with a Fischer-Tropsch catalyst including the particles of thisinvention, which leads to hydrocarbons and water. Depending on theprocess conditions and the catalyst used, the nature of the hydrocarbonsand the chain length may vary. The main products of the Fischer-Tropschreaction are linear olefins and paraffins and water. The third stepinvolves isomerisation of the hydrocarbons formed in the second step toproduce more valuable products. For instance, the longer chains in theproduct may be cracked to form products in the diesel or gasoline range,and linear paraffins may be isomerised to improve diesel productproperties such as cloud point and pour point. Generally, adaptedhydrotreating catalysts are used for this third step.

Whatever the reaction in which they are used, the core-shell particlesof this invention are usually included within and/or adsorbed on aninert porous carrier so as to form a catalyst. This carrier may be aporous inorganic refractory oxide, more preferably alumina, silica,titania, zirconia or mixtures thereof.

Alternatively, it may be chosen from aluminosilicates such as zeolithes.This carrier is usually beneficial to the retention of the core-shellstructure under reducing conditions. Usually, the core-shell particlesare impregnated onto the carrier, which preferably has a pore size of atleast 40 nm. To this end, a particle suspension with a concentration offrom 5 to 20 g/l may be used, and water may be slowly removed afterimpregnation.

The optimum amount of core-shell particles present in the carrier mayvary, depending on the catalytic activity required. Typically, theamount of cobalt present in the catalyst may range from 1 to 25% byweight of catalyst, for instance from 10 to 20% by weight of catalyst.Other promoters, if present, may represent from 0.5 to 5% by weight ofcatalyst. The promoters may be present as metals or as the metal oxide,depending upon the particular promoter concerned. Suitable promotersinclude oxides of metals from Groups IVB, VB and/or VIIB of the PeriodicTable, oxides of the lanthanides and/or the actinides. Preferably, thecatalyst comprises at least one oxide of titanium, zirconium, manganeseand/or vanadium. As an alternative or in addition to the metal oxidepromoter, the catalyst may comprise a metal promoter selected fromGroups VIIB and/or VIII of the Periodic Table. Preferred metal promotersinclude rhenium, platinum and palladium.

According to an embodiment of this invention, the core-shell particlesdescribed above may be used to manufacture a catalyst comprising from 3to 8% by weight of cobalt, from 0.2 to 1% by weight of ruthenium andfrom 0.5 to 1.5% by weight of platinum.

When fresh prepared, the catalyst may be shaped or formed by means ofspray drying, pelletizing, (wheel) pressing, extrusion, or applicationon a metal support like a metal wire. The core-shell particles and/orany promoter may be added to the carrier material before or aftershaping. The catalyst suitably has an average diameter of 0.5-15 mm. Oneform of catalyst is as an extrudate. Such extrudates suitably have alength of 2-10 mm, and a cross section suitably of 1-6 mm², especially2-3 mm².

After shaping, the catalyst may be strengthened by calcination thereofin a manner known in the art. The calcination temperature depends on thecarrier material used.

Activation of a fresh prepared catalyst can be carried out in any knownmanner and under conventional conditions. For example, the catalyst maybe activated by contacting it with hydrogen or a hydrogen-containinggas, typically at temperatures of about 200° C. to 350° C.

The catalyst may then be used as a slurry catalyst or preferably as afixed bed catalyst. For instance, if developed for carrying out theFischer-Tropsch reaction, this catalyst may be used in fixed bedreactors, especially multi-tubular fixed bed reactors, fluidised bedreactors, such as entrained fluidised bed reactors and fixed fluidisedbed reactors, and slurry bed reactors such as three-phase slurry bubblecolumns and ebullated bed reactors.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention will be further understood in light of the followingnon-limiting examples which are given for illustration purposes only,and also in connection with the attached drawings in which:

FIG. 1 is a TEM image showing, on the left side, core-shell particles ofthis invention and, on the right side, platelets of cobalt oxide;

FIG. 2 is a HR-TEM image of a core-shell particle of this invention;

FIG. 3 is a TPR plot of core-shell particles of this invention comparedto magnetite;

FIG. 4 is a TPR plot of core-shell particles of this invention adsorbedon a porous carrier.

EXAMPLES Example 1 Synthesis and Characterization of Core-ShellParticles

Magnetite particles were first synthetized via a slightly modifiedMassart method. To this end, 9.02 g of FeCl₃.xH₂O and 3.26 g ofFeCl₂.xH₂O were mixed together in 380 ml of water. To this solution wereadded from 10 to 40 ml of ammonia: magnetite formation was visible as ablack precipitate. The particles were washed with 300 ml water, untilthe pH of the supernatant was constant. The magnetite particles werethen peptized with 40 ml of a 2M HNO₃ solution. The precipitate wasrecovered with a magnet and redispersed in water.

The following Table 1 summarizes the size of the magnetite particles asa function of the ammonia amount:

TABLE 1 NH₄OH (ml) Particle size (nm) 10 12.2 20 7.6 40 3.5

50 ml of a 20 g/l magnetite suspension were then mixed with 10 ml ofNaOH solution. The mixture was heated up to 70° C. Then 10 ml of aCo(NO₃)₂.xH₂O solution was slowly added to the magnetite suspension atthe speed of 0.2 ml/min. The Co concentration in the solution was chosenso as to achieve a final Co/Fe wt % varying in the 3-60% range.

The precipitate was then washed with water, the supernatant was removedand the so-obtained slurry was freeze-dried. The particle suspensionsshow an extended stability in a wide range of pH values. The isoelectricpoint is at pH=7-8. At pH below 2, the particles dissolve and at pHabove 12 the ionic strength is high and the particles settle quite fast.

Mean iron particle size, calculated on the basis of the TEM imageanalysis, is comprised in the 3-12 nm range and inversely proportionalto the amount of ammonia used in the synthesis. The polydispersity ofthe systems (span=0.6) is in agreement with similar aqueous proceduresreported in the literature. The image analysis of the Co coatedparticles showed no homogeneous nucleated Co oxide particles (see FIG. 1a which illustrates core-shell particles of about 7.5 nm). A controlexperiment further showed how, with no magnetite particles in the hotbasic solution, Co oxide precipitates as hexagonal plates of about 20 nm(see FIG. 1 b).

Moreover, there appeared to be no statistically relevant size differencebetween the pure magnetite particles and the Co-coated ones. Rather, asevident from the EDX analysis, Co-enriched regions are formed on the topof the magnetite particles. High resolution TEM (HR-TEM)characterization also provided visual evidence of this new layer, asshown on FIG. 2 which illustrates core-shell particles with a Co/Feratio of 60 wt %. The Co-rich phase is visible as a lighter region onthe surface of the particles. Furthermore, lattice fringes analysisshowed that core and shell crystal structure are aligned on the samedirection.

TPR characterizations were also performed on freeze-dried powders byflowing a 5% H₂ in N₂ mixture at 40 ml/min, heating the samples at 5°C./min. The TPR characterization as well supports the presence of acore-shell structure. The behaviour in a reducing environment of theseiron oxide particles perfectly matched magnetite TPR profile reported inthe literature. From the TPR plots illustrated on FIG. 3, one couldderive that the part of the Co interacted with the magnetite structure,influencing Fe reducibility, and that part of it also contributed to theformation of the cobalt-rich shell. The reduction rate was so fast inany way that the presence of pure Co₃O4 and Fe₃O4 phases could not bedetected.

Similar TPR plots of these core-shell particles supported on mesoporoussilica (EMS 385 supplied by Eurosupport as extrudates) having a meanpore size of 41 nm (span=0.7), a specific pore volume of 0.47 cm³/g anda specific surface area of 125 m²/g showed, on the contrary, that theCo-rich and the iron-rich phases of the core-shell particles behavedmore closely like pure CO₃O₄ and Fe₃O₄ when increasing the Co/Fe ratio.It could then be derived that the support had a beneficial affect in theretention of the core-shell structure under reducing conditions.

1. A method for the preparation of spherical core-shell particles,comprising the successive steps of: (a) preparing an aqueous solutioncomprising a ferric salt, at a temperature of less than 50° C.; (b)adding at least one base to said aqueous solution, so as to obtain asuspension of iron oxide particles having a pH value of from 10 to 14;(c) washing the suspension; (d) adding a strong acid to the washedsuspension to peptize the washed suspension; (e) reacting at least onebase with said peptized suspension, until the pH reaches a value from 10to 14, at a temperature of from 50 to 95° C.; (f) adding a cobalt saltto the heated suspension in order to obtain spherical particles having acore of iron oxide and a shell comprising cobalt oxide.
 2. The methodaccording to claim 1, characterized in that the aqueous solutioncomprising the ferric salt further includes a ferrous salt in a molarratio of Fe(III) to Fe(II) of 2:1, whereby the iron oxide particles aremagnetite particles.
 3. The method according to claim 1, characterizedin that the ferric salt is ferric nitrate, ferric chloride or ferrichydroxide.
 4. The method according to claim 2, characterized in that theferrous salt is ferrous nitrate, ferrous chloride or ferrous hydroxide.5. The method according to claim 1, characterized in that the cobaltsalt is cobalt nitrate, cobalt chloride or cobalt sulphate.
 6. Themethod according to claim 1, characterized in that the strong acid isnitric acid or hydrochloric acid.
 7. Core-shell particles obtainedableaccording to the method of claim
 1. 8. Core-shell particles comprising acore of iron oxide and a shell of cobalt oxide, characterized in thatthe core-shell particles are spherical with a number average diameter,as measured by TEM, of between 1 and 20 nm.
 9. Core-shell particlesaccording to claim 8, characterized in that the core comprises magnetiteor hematite and the shell consists comprises cobalt oxide.
 10. A methodcomprising manufacturing a catalyst using the core-shell particlesaccording to claim
 7. 11. A catalyst comprising an inert porous carriercontaining the core-shell particles of claim
 7. 12. A catalystcomprising an inert porous carrier containing the core-shell particlesof claim
 8. 13. A method comprising manufacturing a catalyst using thecore-shell particles according to claim
 8. 14. The method according toclaim 3, characterized in that the ferric salt is ferric chloride. 15.The method according to claim 4, characterized in that the ferrous saltis ferrous chloride.
 16. The method according to claim 5, characterizedin that the cobalt salt is cobalt nitrate.